1. Technical Field of the Invention
The present invention relates to a magnetic recording medium comprising a polymer support having thereon a magnetic layer containing a ferromagnetic powder and a binder and further to a magnetic recording medium having an excellent electromagnetic conversion characteristic and reliability.
2. Description of the Related Art
In the magnetic recording field, putting digital recording which is small in deterioration of recording to practical use is developing in place of the conventional analog recording. In recording and reproducing equipment and magnetic recording media which are used in digital recording, not only high image quality and high sound quality, but also miniaturization and space reduction are required. However, in general, since much signal recording is necessary in the digital recording as compared with the analog recording, the digital recording is required to realize recording with a higher density.
In recent years, a reproducing head applying magnetic resistance (MR) as an actuation principle was proposed and began to be used in a hard disc, etc. Also, JP-A-8-227517 proposes to apply the reproducing head to a magnetic tape. In an MR head, since a reproducing output of several times as compared with an induction type magnetic head is obtained and an induction coil is not used, by largely lowering noises of instruments such as an impedance noise to lower a noise of a magnetic recording medium, it becomes possible to obtain a large SN ratio. In other words, if the noise of the magnetic recording medium hidden in a conventional instrument is made small, good recording and reproduction can be achieved, and a high-density recording characteristic can be greatly improved.
So far, in magnetic recording media, ones comprising a support having thereon a magnetic layer having Co-modified iron oxide, CrO2, a ferromagnetic metal powder, or a hexagonal ferrite powder dispersed in a binder are widely used. For the sake of reducing the noise, various measures may be considered. In particular, it is effective to decrease the size of a grain of the ferromagnetic powder. In recent magnetic materials, ferromagnetic metal fine powders having an average major axis length of not more than 100 nm are used, thereby enhancing the effect.
In order to achieve the foregoing high-density recording, it is necessary to realize shorter the wavelength of a recording signal or to make the recording tracks narrow. For achieving this, in addition to realization of fine division of a ferromagnetic powder, high packing and ultra-smoothening of the surface of a magnetic recording medium, it is required to make a magnetic recording medium thin for the purpose of improving the volume density.
In general, a coating type magnetic recording medium has a structure in which a magnetic layer is provided on a support, or a non-magnetic layer and a magnetic layer are provided in this order on a support. For the sake of making the foregoing magnetic recording medium thin, it is required to make not only the magnetic layer but also the whole of layers constructing the magnetic recording medium thin. For the purpose of making the thickness of the magnetic recording medium thin, it has hitherto been carried out to make the support thin or to make the non-magnetic layer thin. However, if the support is made thin exceeding a certain range, the running durability is lowered; and if the non-magnetic layer is made thin, a lowering of the output, an increase of the error rate, and an increase of the dropout are introduced.
That is, if thinning of the magnetic recording medium advances for the purpose of increasing the recording density, a sufficient leveling effect against the support is not obtained in the magnetic layer, and the surface state of the support provided beneath the magnetic layer largely influences the surface of the magnetic layer. It may be considered that the principal cause resides in very small protrusions (so-called fish eyes) scattered on the surface of the support; the fish eyes become an anti-blocking filler, thereby lifting up the surface of the magnetic layer to form protrusions; and the dropout is generated due to these protrusions. In particular, in a linear recording system, since a magnetic tape runs substantially in parallel to a magnetic head and comes into contact with the head, the dropout caused due to protrusions present on the surface of the magnetic layer is liable to be generated.
In order to prevent the dropout caused by the foregoing protrusions on the magnetic layer, it is necessary to change the filler contained in the support and smoothen the surface of the support. However, if the filler contained in the support is changed, the film formation step of a support, the production step of a magnetic recording medium, and the running properties within a drive after forming a tape are greatly influenced, and therefore, it cannot be said that this is an effective method. For this reason, a support having two or more layers in which the surface properties are made different between the side of the support at which a magnetic layer is provided and the side of the back surface against the former.
Further, it is known that even if the support, especially the surface of the magnetic layer is smoothened, a stain is accumulated on the head, resulting in the occurrence of dropout. This is caused by the matter that an edge debris formed when an end face of the support is shaven by a running system within the drive is accumulated, and this end face is generated by slitting.
Now, for the purpose of preventing a poor pancake shape from the occurrence by preventing a high edge of an end portion generated in the slitting step, JP-A-8-45060 describes a magnetic recording medium using a support made of polyethylene naphthalate having a thickness of 4 μm or more and regulated so as to have a ratio of the Young's modulus in the machine direction to the Young's modulus in the transverse direction of from 0.4 to 1.5 and a viscosity of from 0.45 to 0.53.
The foregoing definition of the physical properties of the support is extremely broad and unclear. Also, only the foregoing definition is insufficient as a support for the recent magnetic recording media having an improved recording density. Since this JP-A-8-45060 discloses neither unit nor measurement method regarding the density, its invention is obscure. Also, with respect to the raw material of the support to be used, only the polyethylene naphthalate is described, but no description regarding its layer construction and surface properties is given.
As described above, according to the conventional supports, it is difficult to provide a magnetic recording medium adapted for the recent demand of high recording density.